How do I scale a video clip down without creating a border?
A quick fix for this is to plug the Image value of your Keying node into the Fac input of a ColorRamp, and also into the top Image input of an Alpha Over node. Then plug the Image output of the ColorRamp node into the Fac input of the Alpha Over node. Now plug your background image into the bottom Image input of the Alpha Over node. At this point, everything should look relatively the same as before. Now in order to make the black box transparent, drag the right handle of the ColorRamp node all the way to the left. At this point, your black box should be gone, if it is not just add an Invert node directly after your ColorRamp node, and now the black box should be gone.

Before:


After:


I hope this helped.

-Tom